Photonic crystals are an emerging class of materials, which interact with electromagnetic radiation through a periodic modulation in their refractive indices when the periodicity coincides with the scale of the radiation wavelength (see E. Yablonovitch, Phys. Rev. Lett. 58, 2059 (1987), and. S. John, Phys. Rev. Lett. 58, 2486 (1987)). Though for the moment they are objects of mostly fundamental scientific interest, photonic crystals will likely become increasingly prevalent in both basic and applied circles, furthering their potential for a myriad of proposed and unforeseen applications, (see: J. D. Joannopoulos, P. R. Villeneuve, Nature 386, 143 (1997)).
A particularly elegant method for fabricating these intricately structured materials involves the self-assembly of a collection of size-monodisperse spherical colloids into a long-range ordered lattice of well-defined geometry. Spherical colloids of both inorganic and polymeric composition have been studied exhaustively (see P. C. Hiemenz, R. Rajagopalan “Principles of Colloid and Surface Chemistry”, Marcel Dekker Inc., New York (1997)), can be routinely obtained as monodisperse suspensions, and are quite inexpensive to produce. Given these considerations, and recent success in assembling these colloids into face-centered cubic (fcc) crystals with very low defect concentration, it is not surprising that they have been thrust to the forefront of photonic crystal research (For recent reviews see (a) Y. Xia, B. Gates, Z-Y. Li, Adv. Mater. 13, 409 (2001);(b) V. L. Colvin, MRS Bull. 26, 637 (2001)).
The concept of a self-assembled colloidal photonic crystal, though a new one, has been successfully demonstrated many times, and a great variety of materials have been incorporated into these systems by templating against colloidal crystals or their replicas (arrays of air holes in dielectric media), see B. T. Holland, C. Blanford, A. Stein, Science 1998, 281, 538. Consequently, research in the area has begun to focus on designing systems not only composed of materials unexplored in this regard, but capable of being tuned in a variety of ways by external stimuli, K. Busch, S. John, NATO Sci. Ser., Ser. C: Math. Phys. Sci. 563, 41 (2001). The promise of this approach lies not only in being able to vary optical properties in a very controlled manner, but also in using the intricate light-microstructure interplay that creates structural color (that is, to be contrasted with color from pigments or chromophores) in these colloidal photonic crystal materials to effect accurate sensing with an easily measurable optical response.
One of the ways in which a colloidal photonic crystal can be tuned is via a mechanical response, that is, changing external conditions such that the crystal changes in shape or dimensions. Seminal studies in the experimental demonstration of this concept, using the volume changes of a swollen hydrogel have been made by Asher and coworkers, see J. M. Weissman, H. B. Sunkara, A. S. Tse, S. A. Asher, Science 274, 959 (1996). Their approach was to fix in a hydrogel matrix an array of highly charged latex microspheres self-assembled in a non-close packed array through mutually repulsive electrostatic interactions in a rigorously de-ionized medium.
These so-called polymerized crystalline colloidal arrays (PCCA's) could take advantage of the well-known properties of organic polymer acrylamide-based gels, (T. Tanaka, Phys. Rev. Lett. 40, 820 (1978)) as well as perform sensing functions by incorporating into these organic polymers receptors for specific analytes (J. H. Holtz, S. A. Asher, Nature 389, 829 (1997)). These systems are quite elegant, but suffer from a number of drawbacks such as poor mechanical stability due to high solvent content, slow response to stimuli, and the polycrystalline nature of the samples, which prevents them from being able to accurately control features of the Bragg optical diffraction peak, such as its wavelength, width and intensity.
Some researchers have taken the lead of Asher and co-workers and utilized the PCCA system and post-modified it to achieve a higher mechanical stability, (S. H. Foulger, J. Ping, A. C. Lattam, Y. Ying, D. W. Smith Jr., Adv. Mater. 13, 1898 (2001)) but this system still suffers from the same fundamental drawbacks as the original materials developed by Asher et al. Still others have used the same organic polymer hydrogels employed by these prior researchers and have incorporated them into close-packed colloidal crystals, resulting in a similar type of material simply made by a slightly modified route (Y. Takeoka, M. Watanabe, Langmuir 18, 5977 (2002)). The important point to note about this prior PCCA work is that the matrix that encapsulates the colloidal crystal is an organic polymer gel, which as amplified upon below is quite distinct in its composition and properties to the metallopolymer gels used in the invention described herein.
The materials investigated in the invention described herein are planarized composite colloidal photonic crystals consisting of an ordered face centered cubic (fcc) arrangement of sub-micrometer size microspheres in a matrix of weakly cross-linked polyferrocenylsilane (PFS), whose cross-link density can be continuously and finely adjusted, a novel metallopolymer (also referred to in this invention as an metallopolymer), which has the unique ability to function as a reversibly redox tunable, swellable-shrinkable metallopolymer gel, (K. Kulbaba, M. J. MacLachlan, C. E. B. Evans, I. Manners, Macromol. Chem. Phys. 202, 1768 (2001)). This class of reversibly redox tunable, swellable-shrinkable colloidal photonic crystal metallopolymer gel composite materials are entirely new—composites of this genre have never been made before nor reported before in the patent or open literature. They display unique chemical and physical properties because the metallopolymer gel matrix unlike all known organic polymer gels has metal atoms that are directly integrated into the backbone of the polymer. As a result of a polymer backbone comprised of metal atoms these metallopolymer gels overcome many deficiencies inherent in organic polymer analogues and also introduce additional and valuable functionality by virtue of the properties of the metal-backbone containing polymer used (I. Manners, Science 294, 1664 (2001)). Because this class of redox-active metallopolymers have well-defined, stable redox couples, the state of charge of the metallopolymer can be continuously, finely and reversibly changed in a straightforward manner, chemically or electrochemically, to effect a change in its interactions with a fluid medium. This can result in either a chemo-mechanically or electro-mechanically induced expansion or contraction of the colloidal photonic crystal lattice upon switching the metallopolymer gel to the oxidized or reduced state due to the variety of fluid media available.
Although there has been one previous report dealing with the electrochemical tunability of the optical properties of an inverse colloidal crystal made of tungsten trioxide, (Sumida, T.; Wada, Y.; Kitamura, T.; Yanagida, S. Chemistry Letters 2, 180 (2002)) it is important to note that this system is completely different to the one described in the present invention in that tungsten trioxide is a solid state material and not a polymer and further the tunability of the optical colloidal photonic crystal properties arises from a classical metal-nonmetal induced refractive index change in the tungsten trioxide inverted colloidal photonic crystal matrix that occurs on electrochemically injecting lithium ions and electrons into the void spaces within the structure and conduction band respectively and not a change in colloidal photonic crystal lattice dimensions, which is the basis of the mode of operation of the metallopolymer gel colloidal photonic crystal composite materials in the embodiment described herein.
It would be very advantageous to provide an economical method of producing colloidal photonic crystal materials which are rapidly and reproducibly wavelength tunable.